Iron may be present in hydrocarbon mixtures due to corrosion of the vessels which contain the hydrocarbon. The detection of iron in hydrocarbon mixtures is therefore desirable, as it can act as an indicator of corrosion. However, many of the methods which are known in the art are either inadequate or time consuming.
One area where the detection of iron levels is particularly important is in cylinder oil for combustion engines, particularly marine engines which can be run at lower temperatures. The internal components of engines, particularly the cylinder lining of combustion engines, are subject to degradation due to a combination of corrosion, adhesive and abrasive wear. Under normal operating conditions, corrosive wear contributes most of the total wear of a machine or engine lining.
This corrosion is caused by chemical reactions between the piston rings and/or cylinder lining and acidic products associated with the combustion of sulphur-rich oils.
If the engine temperature is below the dew point of sulphuric acid, the acid can build up on the cylinder liners thereby promoting their “cold corrosion”.
The problem of cold corrosion can be exacerbated by engine and machine operators trying to conserve fuel. For example, more recently ship operators have taken to running diesel engines at reduced speeds (so-called slow steaming) as a means of improving fuel consumption. However, this reduction in speed causes the engine temperature to drop, increasing the levels of sulphuric acid in the oil and accelerating the cold corrosion process.
Corrosion of the piston rings and/or lining of a cylinder in a machine or engine can decrease the efficiency of the machine or engine, and will also result in a build-up of contaminant in the oil or hydrocarbon used for lubricating the cylinders. At its extreme, corrosion reduces efficiency and eventually can damage an engine to such an extent that it can no longer be used. It is therefore desirable to provide methods which are capable of detecting corrosion and particularly cold corrosion at an early stage, so that pre-emptive measures can be used to mitigate further damage.
Corrosion of a machine or engine can be reduced by, for example, increasing the flow of lubricant. However, lubricant can be expensive, and replacing it unnecessarily will often incur significant costs.
It has become standard practice onboard large marine vessels to monitor the used engine lubricant for contaminants, in particular iron, with a view to determining the wear rate of the engine.
The iron found in used cylinder lubricant typically exists in one of three oxidation states, each one having its own particular properties. For example, metallic iron particles worn off the cylinder liner have an oxidation state of 0, which exhibits strong ferromagnetism. In addition to the metallic particles worn off a machine or engine, other corrosion products are found. For example, iron sulphate is a likely product of the reaction between sulphuric acid and the lining of a machine or engine. In contrast to metallic iron, the iron in iron sulphate has an oxidation state of +2, and hence cannot be detected magnetically. The same is true of other corrosion products such as rust, where iron has an oxidation state of +3.
It is possible to detect wear that produces metallic iron particles using a magnetometer, such as the Parker Kittiwake LinerSCAN. However, by the time that magnetometry registers the presence of metallic iron particles, significant damage to the machine or engine may have already occurred.
Previously methods for determining the levels of iron contaminant in used lubricant have involved sending samples of the oil to laboratories relatively far removed from the operational site. However, this process took a long time, during which critical damage to the machine or engine could occur.
Methods are now available that use an on-site method for determining the concentration of iron contaminants, allowing the current corrosion status of the engine to be determined more quickly.
For example, the MobilGard Monitor by ExxonMobil is a portable oil tester that uses the distortion of the magnetic flux field to determine the amount of iron in the 0 oxidation state.
U.S. Pat. No. 4,203,725 relates to an on-site method for determining the need for replacement of oil due to the build-up of metallic contaminant material therein, involving vigorously mixing a known volume of oil in a known volume of aqueous solvent, including a reagent capable of reacting with the metal and/or metal oxide, and comparing the color as generated to a standard color to determine the concentration of the metal contaminant. However, this method takes several hours to test for the concentration of iron in the used oil.
U.S. Pat. No. 4,238,197 relates to a method for analyzing used lubricating oil for iron wear metal content, in which all of the iron in a sample of the oil is extracted into an oil immiscible layer. Buffering and reducing agents are added, and the iron in the solution is reacted with a chelating agent to form a red complex indicative of the iron content. However, this is again a slow process, and uses separate reducing and chelating agents.
The Chevron DOT.FAST drip oil analyser analyses the iron concentration of an oil sample. It uses a solvent to reduce any iron (III) in the oil sample to iron (II) and quantifies the iron (II) content colorimetrically. However, it also uses separate reducing and complexing agents, and measures the total iron content of an oil sample, not just the ferrous and ferric content. It also requires a filtration step, which means the process can take many hours to complete.
Hence whilst there are methods for determining corrosion (particularly cold corrosion) by measuring the iron content of oils, these methods are relatively slow. The present invention seeks to provide fast, effective and simple methods for detecting ferrous and ferric ions in hydrocarbon mixtures. The present invention also seeks to provide methods of detecting the early signs of cold corrosion before any significant damage has been done to the machine or engine.